Petrology and Structural Geology – serie

2Gpa has increased to more than 15. This indicates that subduction of continental fragments to depths of 100-150 km may have played a significant role in the formation of mountain belts. This volume brings together the geochemical, geophysical and geodynamical approaches to study the processes active during ultrahigh-pressure (UHP) tectonics. The collection of papers demarkates the frontier of our understanding of the creation, preservation, and exhumation of ultrahigh-pressure rocks. Audience: This volume will be of interest to any earth scientist interested in ultrahigh pressure processes and the formation and modification of continental crust.

Partial melting occurs in a variety of geological environments, from granitic partial melts in the continental crust, to basaltic or carbonate partial melts in the upper mantle. Partial melting is the first stage of magmatism and therefore plays a role of primary importance in the chemical differentiation of the Earth and in the transport of heat to the Earth surface. This special volume contains contributions presented at the symposium "Physics and Chemistry of Partially Molten Systems" of the EUG 9 meeting, held in Strasbourg, France, on March 23-27, 1997. It is intended to provide a current understanding of the physics of partial melting and melt segregation and covers topics such as the rheology of partially molten systems, the topology of partial melts, modelling of partial melting processes, and field observations of partial melts.

1.1. HISTORICAL DEVELOPMENT OF THE OPHIOLITE CONCEPT. Ophiolite, Greek for 'the snake stone', appears to have received its first written definition by Brongniart (1813) as a serpentine matrix containing various minerals. Later in 1821 and 1827, Brongniart determined that volcanic and gabbroic rocks were also present, associated with cherts, and he ascribed an igneous origin to the ophiolite. Amstutz (1980) gives an excellent exegesis of these early contributions and traces the further use of the term and concept of ophiolite. This concept had been forged in the western Alps and Apennines where, thanks to talented Italian geologists, in particular A. Sismonda, B. Gastaldi, V. Novarese and S. Franchi, the study on metamorphic ophiolites (the 'pietre verdi') has rapidly progressed. At the tum of the century the association of radiolarite, diabase, gabbro (euphotide), and serpentinite-peridotite was clearly identified, even through their metamorphic transformations.In 1902, Franchi developed the hypothesis introduced earlier by Lotti (1886), of a submarine outflow to explain the 'pietre verdi' association, on the basis of the attribution of the variolites and metamorphic prasinites to an hypabyssal volcanism, also responsible for the formation of radiolarites. Thus, before the popular work of Steinmann in 1927, the various components constituting an ophiolite had been identified and its hypabyssal origin proposed. As recalled by Amstutz (1980), the so-called 'Steinmann trinity', which consists of the association of radiolarites, diabases and serpentinites, was more completely and better defined in these earlier works.

Ophiolites are key sources of information regarding the genesis and evolution of oceanic lithosphere. Over the past decades, the geological ~tudy of ophio- lites has provided a wealth of insight into lithospheric processes and has proved to be an indispensible prerequisite to interpreting geophysical and other investigations of the crust underlying recent oceans. The Oman Ophiol- ite offers the most complete and structurally undisturbed sections of the oceanic crust in vast, clean exposures. It is, therefore, most fortunate for the scientific community that Mhd. Kassim, Director General of Minerals and Dr. Hilal Al Azri, Director of the Geological Survey, took upon themselves the task of organizing in Oman an international meeting on ophiolites. Having planned for an attendance of only 100 to 150 persons, the logistics of the organizing committee were put to a severe test by th~ 300 participants who eventually arrived from 27 countries. The 14 field trips, most of which were conducted twice, provided the participants with an excellent introduc- tion to the geology of Oman, the ophIolite sequence, and re~ated phenomena.

This volume follows a Specialized Symposium on "Mantle denudation in slow spreading ridges and in ophiolites", held at the XII EUG Meeting in Strasbourg, spring 1993. During the meeting it was felt that the contribu­ tions to the Symposium justified a volume presenting its main scientific achievements. The present title of the volume shows that the center of inter­ est has slightly shifted with respect to the initial objective: in order to under­ stand the processes involved in accretion taking place at oceanic ridges, it is crucial to study the interaction between uppermost mantle and lower crust. The approach favored here is that of petrological and structural analysis of oceanic rocks in present-day oceanic ridges combined with similar studies in ophiolites. Rock specimen collected by submersibles or dredge hauls in oceanic ridge environments provide a "ground truth". However, except for areas such as the MARK (Mid-Atlantic Ridge ne ar Kane fracture zone) where, thanks to multiple submersible dives, the local geology is known with aprecision even better than in many onshore ophiolites, mutual rela­ tionships between uppermost mantle and lower crust are poorly known. In contrast, onshore ophiolites provide a necessary large-scale picture built up over many years of structural and petrological mapping.

Recent discoveries of diamond and coesite in the ultrahigh-pressure (UHP) metamorphosed supracrustal rocks have provoked a challenge to geodynamic ideas. Chinese geologists have been taking an active role. The Dabieshan-Sulu region, a large and exposed UHP belt, has attracted international attention. This text describes many aspects of the UHP rocks in the Dabieshan-Sulu region, including the geotectonic setting, mode of occurrence, mineralogy and petrology, isotope chronology, major and trace element chemistry, and metamorphic PTt path. The possible geodynamic mechanisms involved in the deep subduction and rapid exhumation of UHP rocks are also discussed.

viii debate of those earlier days has been beautifully summarized by H. H. Read in his famous "Granite Controversy" (1957). Read's formulation of the controversy occurred at the time when geochemistry was as a new and powerful tool. The new techniques opened era during which emerging an granites were considered mainly from this new viewpoint. Geochemical signatures have shown that mantle and crustal origins for granites were both possible, but the debate on how and why granites are emplaced did not progress much. Meanwhile, structural geology was essentially geometrical and mechanistic. In the early 70's, the structural approach began to widen to include solid state physics and fluid dynamics. Detailed structural maps of granitic bodies were again published, mainly in France, and analysed in terms of magmatic and plastic flow. The senior editor of this volume and his students deserve much of the credit for this new development. Via microstructural and petrofabric studies, they were able to discriminate between strain in the presence of residual melt or in the solid-state, and, by systematically measuring magnetic fabrics (AMS), they have been able to map magmatic foliations and lineations in ever finer detail, using the internal markers within granites coming from different tectonic environments. The traditional debate has been shifted anew. The burning question now seems to be how the necessary, large-scale or local, crustal extension required for granite emplacement can be obtained.

Physicists attempt to reduce natural phenomena to their essential dimensions by means of simplification and approximation and to account for them by defining natural laws. Paradoxically, whilst there is a critical need in geology to reduce the overwhelming field information to its essentials, it often re­ mains in an over-descriptive state. This prudent attitude of geologists is dictated by the nature of the subjects being consi­ dered, as it is often difficult to derive the significant parame­ ters from the raw data. It also follows from the way that geolo­ gical work is carried out. Geologists proceed, as in a police investigation, by trying to reconstruct past conditions and events from an analysis of the features preserved in rocks. In physics all knowledge is based on experiment but in the Earth Sciences experimental evidence is of very limited scope and is difficult to interpret. The geologist's cautious approach in accepting evidence gained by modelling and quantification is sometimes questionable when it is taken too far. It shuts out potentially fruitful lines of advance; for instance when refu­ sing order of magnitude calculations, it risks being drowned in anthropomorphic speculation. Happily nowadays, many more studies tend to separate and order the significant facts and are carried out with numerical constraints, which although they are approxi­ mate in nature, limit the range of hypotheses and thus give rise to new models.

Physicists attempt to reduce natural phenomena to their essential dimensions by means of simplification and approximation and to account for them by defining natural laws. Paradoxically, whilst there is a critical need in geology to reduce the overwhelming field information to its essentials, it often re­ mains in an over-descriptive state. This prudent attitude of geologists is dictated by the nature of the subjects being consi­ dered, as it is often difficult to derive the significant parame­ ters from the raw data. It also follows from the way that geolo­ gical work is carried out. Geologists proceed, as in a police investigation, by trying to reconstruct past conditions and events from an analysis of the features preserved in rocks. In physics all knowledge is based on experiment but in the Earth Sciences experimental evidence is of very limited scope and is difficult to interpret. The geologist's cautious approach in accepting evidence gained by modelling and quantification is sometimes questionable when it is taken too far. It shuts out potentially fruitful lines of advance; for instance when refu­ sing order of magnitude calculations, it risks being drowned in anthropomorphic speculation. Happily nowadays, many more studies tend to separate and order the significant facts and are carried out with numerical constraints, which although they are approxi­ mate in nature, limit the range of hypotheses and thus give rise to new models.

2Gpa has increased to more than 15. This indicates that subduction of continental fragments to depths of 100-150 km may have played a significant role in the formation of mountain belts. This volume brings together the geochemical, geophysical and geodynamical approaches to study the processes active during ultrahigh-pressure (UHP) tectonics. The collection of papers demarkates the frontier of our understanding of the creation, preservation, and exhumation of ultrahigh-pressure rocks. Audience: This volume will be of interest to any earth scientist interested in ultrahigh pressure processes and the formation and modification of continental crust.

This volume follows a Specialized Symposium on "Mantle denudation in slow spreading ridges and in ophiolites", held at the XII EUG Meeting in Strasbourg, spring 1993. During the meeting it was felt that the contribu­ tions to the Symposium justified a volume presenting its main scientific achievements. The present title of the volume shows that the center of inter­ est has slightly shifted with respect to the initial objective: in order to under­ stand the processes involved in accretion taking place at oceanic ridges, it is crucial to study the interaction between uppermost mantle and lower crust. The approach favored here is that of petrological and structural analysis of oceanic rocks in present-day oceanic ridges combined with similar studies in ophiolites. Rock specimen collected by submersibles or dredge hauls in oceanic ridge environments provide a "ground truth". However, except for areas such as the MARK (Mid-Atlantic Ridge ne ar Kane fracture zone) where, thanks to multiple submersible dives, the local geology is known with aprecision even better than in many onshore ophiolites, mutual rela­ tionships between uppermost mantle and lower crust are poorly known. In contrast, onshore ophiolites provide a necessary large-scale picture built up over many years of structural and petrological mapping.

viii debate of those earlier days has been beautifully summarized by H. H. Read in his famous "Granite Controversy" (1957). Read's formulation of the controversy occurred at the time when geochemistry was as a new and powerful tool. The new techniques opened era during which emerging an granites were considered mainly from this new viewpoint. Geochemical signatures have shown that mantle and crustal origins for granites were both possible, but the debate on how and why granites are emplaced did not progress much. Meanwhile, structural geology was essentially geometrical and mechanistic. In the early 70's, the structural approach began to widen to include solid state physics and fluid dynamics. Detailed structural maps of granitic bodies were again published, mainly in France, and analysed in terms of magmatic and plastic flow. The senior editor of this volume and his students deserve much of the credit for this new development. Via microstructural and petrofabric studies, they were able to discriminate between strain in the presence of residual melt or in the solid-state, and, by systematically measuring magnetic fabrics (AMS), they have been able to map magmatic foliations and lineations in ever finer detail, using the internal markers within granites coming from different tectonic environments. The traditional debate has been shifted anew. The burning question now seems to be how the necessary, large-scale or local, crustal extension required for granite emplacement can be obtained.

Ophiolites are key sources of information regarding the genesis and evolution of oceanic lithosphere. Over the past decades, the geological ~tudy of ophio- lites has provided a wealth of insight into lithospheric processes and has proved to be an indispensible prerequisite to interpreting geophysical and other investigations of the crust underlying recent oceans. The Oman Ophiol- ite offers the most complete and structurally undisturbed sections of the oceanic crust in vast, clean exposures. It is, therefore, most fortunate for the scientific community that Mhd. Kassim, Director General of Minerals and Dr. Hilal Al Azri, Director of the Geological Survey, took upon themselves the task of organizing in Oman an international meeting on ophiolites. Having planned for an attendance of only 100 to 150 persons, the logistics of the organizing committee were put to a severe test by th~ 300 participants who eventually arrived from 27 countries. The 14 field trips, most of which were conducted twice, provided the participants with an excellent introduc- tion to the geology of Oman, the ophIolite sequence, and re~ated phenomena.

Partial melting occurs in a variety of geological environments, from granitic partial melts in the continental crust, to basaltic or carbonate partial melts in the upper mantle. Partial melting is the first stage of magmatism and therefore plays a role of primary importance in the chemical differentiation of the Earth and in the transport of heat to the Earth surface. This special volume contains contributions presented at the symposium `Physics and Chemistry of Partially Molten Systems' of the EUG 9 meeting, held in Strasbourg, France, on March 23-27, 1997. It is intended to provide a current understanding of the physics of partial melting and melt segregation and covers topics such as the rheology of partially molten systems, the topology of partial melts, modelling of partial melting processes, and field observations of partial melts. Audience: This book is intended for a broad readership, including graduate students, specializing in petrology and geodynamics. The volume may be recommended as a textbook for graduate courses on petrology, geomaterial sciences and geophysics.